This invention relates to a removable attachment for overhead projectors which permits the operator of the projector to readily control the image display. More particularly, the invention is directed to an attachment for overhead projectors which is transportable for use on different projection heads.
In audiovisual presentations it has become commonplace to use an overhead projector wherein transparencies are placed on a light box contained in the projector base. The images on the transparencies are projected upwardly from the base and reflected by an elevated projection head onto a display surface. The projection head typically includes the combination of a condensing lens and mirror. In general, the displayed images are in aid of an oral presentation and are continually changed to illustrate and emphasize certain points being made by the speaker during the presentation. Normally, the presentation is interrupted to receive comments from the audience. In order to then concentrate attention on the speaker, either the transparency is removed or the overhead projector is switched off. When the lecturer leaves the light on, the removal and replacement of transparencies distracts the audience from the lecturer. Turning the projector off manually interferes with the presentation of the speaker, especially if he is the operator of the projector since the continuity of presentation and continuous eye contact are interrupted. In addition, the frequent use of the on/off switch subjects the light sources to repeated thermal stress and shortens the lifetime thereof.
These problems can be dealt with by furnishing a relatively simple handoperable device which interrupts the projection of the image. One approach to this type of device is set forth in U.S. Pat. No. 4,523,823 wherein a flip panel of opaque material is secured to the front edge of the projection head by a hook and eye fabric fastener such as the flexible fastener sold under the mark VELCRO. The prior art device is constructed so that the panel has two positions, one of which is a blocking position wherein it hangs down over the face of the projection head and the other is a rearwardly projecting position which removes it from the light path. In order to utilize this device, a portion of the fastener has to be adhered to the top of the projection head. This is not always practical since the topmost element of the head is likely to be merely a planar reflecting surface of limited thickness. Furthermore, the device is not usable with different heads since the fastener is not removably secured to the projection head. In addition, the plastic panel when it is in its light blocking position continually receives incident energy thereby raising its temperature over a period of time. The heat generated thereby can cause the panel to deform and impair its usefulness.
Accordingly, the present invention is directed to an attachment device which can be readily removed from a projection head and transported to another site for use with another projection head. In addition, this invention incorporates a blocking panel that is capable of withstanding high temperatures which is important in situations wherein the panel is in a blocking position for an extended period. Furthermore, the subject invention is constructed in a manner which enables the device to be firmly secured to a variety of different size and style projection heads.